Photonic metasurfaces for integrated optical trapping applications

Abstract

Through optical trapping, we aim to replicate the ease and precision of macroscopic level manipulation, such as holding, observing, squeezing, rotating, and probing biological specimens in microfluidic environments. Optical beams access these environments through narrow apertures of bulky and expensive microscopic objectives. These limitations increase the complexity and cost of optical trapping, keeping it out of clinical settings.

To address these constraints, this thesis presents a new biophotonic platform integrating nano- and micro-fabricated optical elements into the microfluidic environment. Arrays of custom parabolic micromirrors are rapidly patterned into glass using CO₂ laser ablation and used to form optical traps. Holographic metasurfaces, flat optical elements capable of arbitrary photonic response, operating in reflection and transmission are used to create optical traps with an equivalent efficiency to commercial microscope objectives. The metasurfaces are then used to generate photonic landscapes with multiple trapping sites without the need for diffractive optical elements.

Very stable 15 x 15 um² square polymeric membranes are fabricated and decorated with handles for optical manipulation and mirrors. This creates a steerable, microscopic mirror, allowing full control over the delivery and collection of light around samples in the microfluidic chamber without the need for multiple microscope objectives. This is, in turn, used for refractive index sensing by selective planar excitation of a whispering gallery mode laser.

This cross-disciplinary project bridges photonics, material sciences, and biology, enabling the adoption of advanced photonic designs in microfluidic environments, with transformative benefits for microscopy and biophotonic applications at the interface of molecular and cell biology.

Date of Award	30 Jun 2025
Original language	English
Awarding Institution	University of St Andrews
Supervisor	Andrea Di Falco (Supervisor)